The invention relates to a process for recovering the overspray of aqueous coating agents during spray application in spray booths, whereby the overspray can be recycled to the aqueous coating agent used in the spray booth.
For known reasons of environmental protection it is becoming ever more important to substitute water-dilutable systems for lacquers diluted with organic solvents. By using waterdilutable systems, pollution of the air by solvent emissions is reduced or entirely eliminated. However, problems arising during the disposal of the overspray (lacquer mist) constitute a general weakness when processing coating agents such as water-dilutable lacquers by spray application; these problems are particularly frequent with large-scale production lacquering as in the automobile industry.
It is known how to clean the exhaust air from spray booths in order to dispose of the overspray by washing with water. DE-A1-29 45 523, for instance, describes a process in which the overspray of conventional solvent-dilutable lacquers is washed with water, whereupon the washing water is subjected to ultrafiltration.
DE-A1-32 27 227 describes a similar system for disposing of the overspray produced when lacquering with aqueous lacquers. The overspray is absorbed on spray booth walls sprinkled with water and the circulated sprinkling water is processed by passing it through an ultrafilter. However, the overspray collected in the sprinkling water has proved not to be suitable for further use in coating agents and must therefore be disposed of. Although ultrafiltration is used when purifying effluent and in particular when removing undesirable low-molecular substances in the course of electrophoretic lacquering processes, thus improving the degree of utility by recycling (DE-C2-21 58 668, DE-B2-22 29 677, EP-A1-0 127 685, EP-A1-0 137 877, U.S. Pat. Nos. 3,663,405 and 3,663,406), the use of ultra-filtration entails problems as regards the recycling of overspray. For whereas ultra-filtration of electrophoretic lacquers enables the return of the permeate and the retentate into the bath material, the different method of application (spraying instead of electro-dipping) and the larger differences in viscosity caused thereby gives rise to stability problems such as possible coagulation, sedimentation, phase separation and precipitation, when recycling the overspray of aqueous lacquers. DE-A1-34 28 300 recommends, with a view to avoiding the stability problems associated with the process as described in DE-A1 32 27 227, sprinkling of the spray booth with desalinated water. However, it has proved that even if desalinated water is used by way of sprinkling liquid it is not possible to overcome the stability problems associated with the overspray absorbed so as to make it suitable for re-use as part of the lacquering material. WO 91/09666 attempts to solve the stability problems associated with the process according to DE-A1-32 27 227 by sprinkling the spray booth with water to which an anti-coalescent agent has been added. By way of such agents use may be made of amines. This means that on the one hand environmentally harmful substances are added while on the other hand the composition of the lacquer is modified the addition of chemicals.
Moreover, raising the concentration of the overspray in water by ultrafiltration from low to higher values gives rise to considerable changes in the aqueous coating agent inasmuch as due to the ultrafiltration certain constituents, in particular water-soluble and low-molecular constituents such as solvents and neutralisation agents, are removed which significantly contribute to the stabilisation of the aqueous lacquer dispersions and their technical lacquer properties. In "Die Na.beta. Blackierung ohne Emissionen oder Sondermull in oberflache+JOT 1991, issue 3, pages 18 to 20, Dirk Saarbach and Georg Schlumpf describe the lacquering of office furniture subject to aqueous-lacquer recycling, specifying also a process of continuous circulation. It is mentioned that the partial systems "water-dilutable stoving lacquer", "ultrafiltration apparatus" and "spraying system" must be in tune with one another. The introduction of additives into the recycling process is also described. However, there is no indication of how to operate the system without incurring stability problems; furthermore, the introduction of additives causes the composition of the original lacquer to be modified.
Only lacquers which do not coagulate or develop other instability phenomena which modify technical lacquer properties when the concentration is raised by ultrafiltration from low to high values are suitable for this process.